Short term storage of natural gas

ABSTRACT

In a process of storing natural gas on a short term basis, comprising liquefaction of the natural gas during low demand periods, wherein a liquid cryophore is employed to store heat liberated during the cooling and liquefaction of the natural gas, and to release heat during the vaporization and warming up of the natural gas, said cryophore being present in the liquid state in every phase of the process, THE IMPROVEMENT COMPRISING EMPLOYING SAID LIQUID CRYOPHORE IN A GIVEN AMOUNT BELOW A MEDIAN TEMPERATURE LEVEL DURING THE COOLING AND LIQUEFACTION, AND DURING THE VAPORIZATION AND WARMING OF THE NATURAL GAS; AT ABOVE SAID MEIDIANTEMPERATURE LEVEL, MEDIAN TEMPERATURE SAID LIQUID CRYOPHORE IN SUBSTANTIALLY LESS THAN SAID GIVEN AMOUNT, AND AT ABOVE SAID MEDIAN TEMPERATURE OPTIONALLY EMPLOYING A SOLID CRYOPHORE, SAID MEDIAN TEMPERATURE BEING APPROXIMATELY THAT TEMPERATURE AT WHICH SAID NATURAL GAS COMMENCES TO LIQUEFY.

United States Patent [1 1 Becker Sept. 25, 1973 Linde A.G., l-Iollriegelskreuth, Germany Filed: Oct. 18, 1971 Appl. N0.: 189,912

Assignee:

[30] Foreign Application Priority Data [56] References Cited UNITED STATES PATENTS 3,058,315 10/1962 Schuftan 62/52 3,661,542 5/1972 Collins ..62/52X FOREIGN PATENTS OR APPLICATIONS 1,153,011 8/1963 Gennany 62/52 1,245,999 8/1967 Germany Primary Examiner-Meyer Perlin Assistant ExaminerRonald C. Capossela Att0rneyl. William Miller et al.

[5 7] ABSTRACT In a process of storing natural gas on a short term basis, comprising liquefaction of the natural gas during low demand periods, wherein a liquid cryophore is employed to store heat liberated during the cooling and liquefaction of the natural gas, and to release heat during the vaporization and warming up of the natural gas, said cryophore being present in the liquid state in every phases? the process, the improvement comprising employing said liquid cryophore in a given amount below a median temperature level during the cooling and liquefaction, and during the vaporization and warming of the natural gas; at above said mediantemperature level, median temperature said liquid cryophore in substantially less than said given amount, and at above said median temperature optionally employing a solid cryophore, said median temperature being approximately that temperature at which said natural gas commences to liquefy.

16 Claims, 5 Drawing Figures PATENTEDSEPZSIQB 3.760.597

sum 2 or 5 KcaI/Nm Fig. 2

PATENTEDSEPZSHJB 3,760,597

SHEU 3 of 5 Fig. 3

PATENTEUsEP25 I975 SHEU 0F 5 Figi SHORT TERM STORAGE OF NATURAL GAS BACKGROUND OF THE INVENTION This invention relates to a system for the liquefaction and revaporization of natural gas or methane during the storage thereof, and in particular to a system wherein the heat liberated during the cooling and liquefaction of the natural gas or methane is stored, and again made available during the vaporization and warming up of the natural gas. An auxiliary coldcarrying medium hereinafter defined as a cryophore serves for heat storage purposes, and is present in the liquid state in each phase of the system.

It is conventional to store excess natural gas available during the summer period at sites near the consumer nitrogen being employed as the cryophore and supply this excess to the consumers during the wintertime by revaporization (DAS [German Published Application] 1,053,01 1, Column 1, lines 3947). Since it is impractical to store gaseous nitrogen cryophore during the period between the liquefaction and the revaporiza tion of the natural gas because of the extraordinarily large tank volume required for this purpose, the nitrogen must always be produced on demand by an air separation stage coupled with the natural gas revaporization stage. Thus, the initial investment and operating costs for the air separation plant may be a burden on the natural gas storage facility, depending on whether the oxygen, simultaneously obtained, can be profitably utilized or sold. I

This economic squeeze can be avoided, however, by the use of pumpable hydrocarbon cryophores employed in the liquid phase during the entire liquefaction and revaporization cycle, as has become conventional by DAS 1,245,999. The heat exchange between natural gas or methane and liquid cryophores is, however, encumbered with considerable energy losses due to the large temperature differences resulting from the fact that the specific heat of the cryophore remains practically constant during the entire course of the process.

SUMMARY OF THE INVENTION It is an object of this invention to reduce the energy losses, occurring when the heat liberated during the cooling and liquefaction of the natural gas or methane is stored, or when this heat is again made available during the vaporization and warming of the natural gas, and when liquid cryophores are used during this process.

Another object is to provide a process especially applicable to peak-shaving demands on a short term basis, especially during a 24 hour period.

Still another object is to provide apparatus for th processes of this invention.

Upon further study of the specification and appended claims, other objects and advantages of the present invention will become apparent.

The above objects are attained, according to this invention, by utilizing a cryophore for heat absorption and heat emission during the cooling and liquefaction, as well as during the vaporization and warming of the natural gas or methane. Below a median temperature determined by the onset of liquefaction of the natural gas or methane, the cyrophore is employed in the liquid state in every phase of the process. Above the median temperature, either the liquid cryophore is employed in a reduced amount or a solid storage substance is utilized.

A main advantage of this process is that the amount of heat given off and/or absorbed by the cryophore per temperature unit, in each temperature range, is dimensioned in such a manner that the heat can be transferred using small temperature differences (AT) to the heat-absorbing or heat-emitting natural gas or methane, even though the specific heats of the aforesaid gases vary greatly with temperature. Consequently, it is possible to conduct the heat exchange between the natural gas or methane and the cryophores with low energy losses.

The process of the present invention is particularly suitable for compensating for short-term fluctuations in demand, for example to shave the peaks during a 24 hour period, there being a lower demand at night than during the day. To accomplish this, excess natural gas or methane is liquefied at night by heat exchange with cold cryophore and then vaporized duringthe day by the absorption of heat from the cryophore. Another short-term storage problem solved by this invention is in the meeting of demands during particularly cold weeks. In this case, the natural gas is liquefied during the weekend and revaporized during the weekdays.

The temperature at which the amount of heat to be transferred to the cryophore per temperature unit during natural gas liquefaction is decreased, or at which the amount of heat to be transferred from the cryophore during natural gas vaporization is increased, is at the onset of liquefaction of the natural gas or methane. This definition is not only to designate the temperature at which, during the course of the cooling step, the formation of a liquid phase begins, but is also to include the temperature at which, during cooling under supercritical pressure, the specific heat of the natural gas or methane corresponding to the onset of liquefaction increases substantially. Whereas the median temperature is preferably the actual dew point, nevertheless considerable advantages are still obtained if the median temperature varies, plus or minus 30 l(, preferably not more than :10 K from the actual dew point temperature.

The materials serving as liquid cryophores according to this invention must exhibit a melting point below that of the liquid natural gas or methane, or at least below 132 K. Besides, it is advantageous to provide that th vapor pressure at ambient temperature about 293303 K. is 1 bar or more (eg. 1 40 bars preferably 10 20 bars). Especially suitable, cryophores include but are not limited to the following hydrocarbons or halogenated hydrocarbons:

Melting Boiling Point Point at Name Fonnula in K 1.01 bar Chlorodifluoromethane CHCIF, 1 13.2 232.4

Dichlorodifluoromethane CCI F 1 18.2 243.4 1-Chloro-1,2,2-trifluoro ethylene C,C1F;, 1 15.7 245.3

l-Chloroethylene C,H C1 1 19.5 259.4

Dimethyldiborane C H ,B, 123.0 270.6

Propylene C H 88.2 225.5

Propane C H 86.1 231.1

l-Methylpropane C H 128.2 261.5

Isoprene C H 126.5 305.8

Z-Methylbutane C 14,, 1 13.5 300.8

Methylcyclopentane C l-1,, 130.8 345.0

Z-Methylpentane s u 1 19.2 333.5

It can be seen from the above table that 2- methylbutane fulfills the aforementioned conditions especially well. Although propane has a very low melting point, it exhibits a rather high vapor pressure at ambient temperature, so that the tank intended for the storage of the auxiliary cold-carrying medium at ambient temperature must be designed for a correspondingly high pressure.

As mentioned above, it is possible, according to this invention, also to employ a solid material as a cryophore above the temperature of the onset of liquefaction. By this combination of storing refrigerant values in regenerator packing and also in a liquid cryophore, the cooling of the natural gas to about the liquefaction temperature and the warming of the vaporized natural gas is accomplished by the regenerator. Since the cryophore essentially stores only the latent heat of the natural gas, this process affords the additional advantage that CO and H need not be removed from the natural gas to be liquefied in a separate plant. Instead, these impurities are deposited during the cooling step on the packing and are absorbed and removed by the natural gas during the warming-up stage. Additionally, it is possible to use, as a cryophore in this system, liquids having a higher vapor pressure, such as, for example, ethane, since there is no need to store the corresponding liquids at ambient temperature.

For the natural gas or methane to be capable of reabsorbing (during the vaporization or warming step) the heat previously transmitted, to the cryophore during cooling and liquefaction, it must be under a higher pressure during liquefaction than during revaporization. The pressure at which the natural gas is fed to the liquefaction plant must, furthermore, be so high that sufficient cold can be produced, in order to compensate for heat exchange losses. This is accomplished by passing the gas through an expansion valve or an expansion engine after heat exchange with the liquid cryophore.

The ratio of the amount of cryophore liquid required for the heat exchange at below the median temperature to the amount required for the heat exchange at above the median temperature depends on the composition of the gas to be treated and on the ratio between the pressures under which the liquefaction and the revaporization occur. In the case of Algerian natural gas (composition approximately 83 mole percent of CH 7.1 mole percent of C H 2.3 mole percent of C l-I 1.4 mole percent of C and higher hydrocarbons, 6 mole percent of N He, 0.2 mole percent of CO the process is preferably conducted with a pressure ratio of about 50 bar: 8 bar; the aforementioned ratio of the required amounts of liquid cryophore measured in cubic meters of liquid is in this case about 2.75 1. In case of pure methane, the liquefaction is preferably conducted under a pressure of 25 bar, and the revaporization is effected under a pressure of 8 bar; the above quantitative ratio is then most favorably about 7 1. In general, however, the latter ratio will be in both cases in the range ofabout l2: 1 to 2: 1, preferably 6: l to 8: 1.

Preferred embodiment of the invention comprises operating in the following manner: During the cooling and liquefaction of the methane or natural gas under an increased pressure, the liquid cryophore is stored at a low temperature but above (e.g. l0 50 K) the storage temperature of the liquid natural gas or methane. This liquid cryophore is heated to the median temperature in heat exchange with the natural gas or methane and is then, in part, (e.g.67 93 percent, preferably 88 73 percent) stored at this temperature. The remainder of the liquid cryophore is warmed to approximately ambient temperature by heat exchange with the natural gas or methane and then likewise stored. During the vaporization and warming of the natural gas or methane and heat exchange therewith, the liquid cryophore which has been stored at the ambient temperature, is cooled to the median temperature. This cooled cryophore together with the liquid cyrophore already stored at the median temperature, is further cooled to said low temperature by heat exchange with the natural gas or methane, and then stored.

In the process of this invention, liquid cryophore must be stored at two different temperatures considerably below ambient temperature, the first at the low temperature just above the storage temperature of the natural gas or methane, and the second, at the median temperature given by the onset of liquefaction of the natural gas or methane. For this purpose, two separate storage tanks can be provided. However, if, in a more sophisticated embodiment of this invention, the liquid cryophore stored at the low temperature is maintained separate from the liquid stored at the median temperature by means of a floating insulating layer, only a single, correspondingly larger tank is required for storing both quantities of liquid.

The hydrocarbons and halogenated hydrocarbons listed as liquid cryohpores exhibit a very low vapor pressure at the median temperature as well as at the low storage temperature. For example, with the use of pure ethane as the cryophore, a pressure of 0.1 atmosphere absolute is obtained at 150 K, and a pressure of 1;] atm. abs. at 185 K; when using ethylene as the cryophore, a pressure of 0.28 atm. abs. is ambient in the tank at 150 K., and a pressure of 2.4 atm. abs. at 185 K. Thus, a tank intended for receiving the cryophore at the low temperature must be designed so that a subatmospheric pressure of, for example, 0.1 atm. abs. can be maintained therein.

The use of storage vessels of sufficient strength to withstand vacuum conditions can be avoided by providing, in a further embodiment of this invention, that an inert gas atmosphere, for example of methane, natural gas, or nitrogen, be maintained in the vapor space above the liquids stored at the aforementioned temperatures, and a slight excess pressure is produced in this connection. Due to the fact that hydrogen is only slightly soluble in liquid hydrocarbons, it is especially suitable as the inert gas. In order to prevent the inert gas from coming into direct contact with the liquid cryophore where it might be dissolved, it is advantageous to keep the liquid cryophore separated from the inert gas atmosphere by means of a cover floating on the surface of the liquid. This floating cover can be, for example, a cork plate or an aluminum plate provided with floats.

Another way to avoid using vacuum withstanding storage means is to employ a liquid mixture of ethylene and methane as the cryophore having a preferred concentration of M01 percent ethylene. When, during night operation, liquid is withdrawn from the storage vessel at the low temperature, a mixture rich in methane will evaporate therein, due to the pressure drop caused by the withdrawal of liquid. Since the amount of liquid, measured in moles, is very large as compared to the vapor space, the liquid will be cooled off only to an insubstantial extent, and also the composition thereof will hardly change. In the vessel operated at the median temperature, the pressure rises due to the introduction, by pumping, of the liquid, and the vapor present therein, which is relatively rich in ethylene, is condensed. A higher methane content increases the pressures in the tank at the low temperature as well as in the tank at the median temperature.

Even more suitable than methane-ethylene is the mixture of methane-ethane as the cryophore having a preferred concentration of 80 95 Mol percent ethane. Additional examples for mixtures of cryophores are methane-propane and methane-butane. In general, it is to be noted, in this connection, that the lower-boiling component of such a mixture of cold-carrying media must be selected to that the vapor pressure of this component is one atmosphere absolute or above, at a temperature somewhat, e.g. l50 above the storage temperature of the methane or natural gas, thus, e.g. at 150 l(.

As the lower-boiling component, argon can be used instead of methane. The use of such cryophore mixtures offers the advantage that the containers need not be designed to be vacuum-proof, and at the same time, no special inert gas tanks and inert gas flow paths are necessary.

During natural gas liquefaction, the amount of liquid cryophore stored at the low temperature is reduced, whereas this amount increases during the natural gas vaporization. Thus, when the storage tanks used for receiving liquid cryophore at the median temperature and at the low temperature are maintained under pressure with the aid of an inert gas, the volume available to the inert gas correspondingly changes. These fluctuations are compensated for by a heat accumulator and a gasholder. In a further embodiment of the invention, this is accomplished by cooling inert gas, during the cooling and liquefaction of natural gas or methane, from the ambient temperature to approximately the median temperature in heat exchange with the liquid cryophore to be warmed to ambient temperature. This cooled inert gas, together with inert gas displaced during the introduction of the liquid cryophore warmed to the median temperature into the associated storage tank, from the vapor space above said liquid, is further cooled to said low temperature. The resultant cooled inert gas is then introduced into the vapor space of a further storage vessel from which the liquid cryophore stored at this temperature is withdrawn. Furthermore, during the vaporization and warming of liquid natural gas or methane, the inert gas displaced during the feeding of the liquid cryophore cooled to the low temperature into the further storage tank, from the vapor space above this liquid, is warmed to the median temperature in heat exchange with the cyrophore to be cooled; This inert gas is, in part, then further warmed to ambient temperature, while the residual portion is fed to the vapor space of the storage vessel from which the liquid cryophore stored at the median temperature is withdrawn.

The plant for conducting the process of this invention comprises a heat exchanger having a flow path for natural gas or methane and a flow path for liquid cryophore, a storage tank for liquid natural gas or methane which can be placed into communication with the cold end of the natural gas flow path via an expansion device or via a liquid pump, and three storage tanks for liquid cryophore, of which the first can be connected, via a liquid pump or a valve, to the cold end of the cryophore flow path, the second, via a liquid pump or a valve, with a central section of the cryophore flow path, and the third to the warm end of the cryophore flow path.

In order to be able to maintain an inert gas atmosphere in the vapor spaces of the first and second storage tanks, the above described apparatus must be supplemented by an inert gas tank which is connected, via the warm end of an inert gas flow path with the vapor spaces of the first and second storage tanks, wherein the vapor space of the first storage tank is connected to the cold end of the inert gas flow path and the vapor space of the second storage tank to a central section of the inert gas flow path.

If the sensible heat of the natural gas or methane is stored by means of a solid storage material, e.g. packing, rather than with the aid of liquid cryophore, an apparatus is provided wherein the inlet pipeline is connected, via a regenerator, the first flow path of a heat exchanger, and a liquid pump or an expansion device, with a storage tank for liquid natural gas or methane, and wherein the second flow path of the heat exchanger is connected, at the cold end, via a liquid pump or a valve, to a container for the liquid cryophore stored at the low temperature and, at the warm end, to a container for the liquid cryophore stored at the median temperature. A heating coil is preferably installed within the regenerator to heat and withdraw the methane which escapes in the gaseous phase from the storage tank, and simultaneously to cool incoming compressed methane or natural gas.

BRIEF DESCRIPTION OF DRAWINGS The process and apparatus of this invention will now be explained in greater detail with reference to the drawings wherein:

FIG. 1 is a schematic flowsheet of preferred rather basic embodiment of the invention showing three storage tanks and temperature levels for liquid cryophore.

FIGS. 2 and 3 are enthalpy temperature diagrams illustrating the efficiencies obtained by the invention.

FIG. 4 is a schematic flowsheet, an embellishment of the process of FIG. 1, depicting the use of an inert gas in conjunction with the liquid cryophore.

FIG. 5 is a schematic flowsheet illustrating the combined use of a liquid cryophore according to the invention in cooperation with the use of a solid cryophore in the form of a regenerator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, at night, the natural gas, freed of CO and H 0 is first compressed from 8 bar to 50 bar in the compressor C and is then cooled in heat exchanger E, to such an extent that the heavy hydrocarbons are condensed out. The latter are removed from the plant via the separator D The portion remaining in the gaseous phase is cooled to l35K and liquefied in heat exchanger E and expanded into the storage tank T wherein the liquid natural gas is stored at a temperature of about 112K undera pressure of 1 bar. The vapor produced during the expansion is warmed, just as the portion of the liquid natural gas vaporized by ambient heat, via the conduit F, in heat exchanger E compressed in compressor C to the pressure under which the natural gas is supplied, then mixed with the latter, and again cooled and liquefied.

The cold required for this process is withdrawn from the liquid cryophore stored in vessel 1; this liquid cryophore is introduced, by means of the pump P,, at a temperature of about l33K to the cold end of the heat exchanger E,, and warmed countercurrently to the natural gas to be cooled and liquefied, to a median temperature of about 215K. At this point, a portion of the cryophone is fed into the storage vessel 2, whereas the remainder is further warmed to ambient temperature and introduced into the storage vessel 3.

The enthalpy-temperature diagram of the heat exchanger E, is illustrated in FIG. 2. The cooling curve of the natural gas under a pressure of 50 bar is denoted by l, the curve for the media to be warmed, namely propane as the cryophore, and the gas withdrawn by conduit F is denoted by ll. it can be seen that, due to the fact that only a portion of the cryophore is warmed to ambient temperature and the remainder is stored at the median temperature, it is possible to adapt the warming curve very well to the course of the cooling curve, so that the heat exchange can be conducted with only minor energy losses. For comparison purposes, this diagram also includes the isobars for 1 bar (long dashed lines) and 100 bar (short dashed lines).

During the vaporization period, i.e. during the day, liquid natural gas is brought to a pressure of 8 bar by means of the pump P The liquid now present in the subcooled condition is warmed in heat exchanger E vaporized, and further warmed to ambient temperature. Countercurrently thereto, the cryophore from the storage tank 3 is first cooled to the median temperature and then, together with cryophore from the storage tank 2 fed via pump P is further cooled in E to a temperature of about I32K which is only above the storage temperature of the liquid natural gas. At this temperature, the subcooled liquid is expanded into the vessel 1.

FIG. 3 shows the enthalpy-temperature diagram of the heat exchanger E wherein III is the cooling curve of the cryophore, and IV is the warming curve of the natural gas under a pressure of 8 bar. The relatively close approach of the cooling curve to the warming curve, especially above about 215K, clearly demonstrates the advantage of employing a larger amount of cryophore larger below a median temperature than above this temperature.

The cold losses of the process are compensated for by the Joule-Thomson expansion of the natural gas from 50 bar to 8 bar.

Referring to FIG. 4, the use of hydrogen is preferred to maintain a slight superatmospheric pressure in the vapor spaces of those storage vessels receiving the liquid cryophore at the median temperature and at the low temperature, respectively. Hydrogen is selected as the inert gas because its equilibrium concentration in liquid propane, as the cryophore is very low. This concentration is, at 132K, about 0.02 mole percent and, at 215K, about 0.04 mole percent.

For precautionary purposes when hydrogen is used, and especially if a gas other than hydrogen is employed (i.e., a gas having a higher equilibrium concentration in the cryophore) for producing an inert gas atomosphere in the storage tanks 49 and 54, it is advantageous to provide floating covers 86 and 87 within the storage tanks 49 and 54 and between the level of the liquid and the inert gas atmosphere, in order to prevent penetration of the inert gas into the liquid. These covers can be made of cork or aluminum sheets.

The nocturnal flow path (natural gas liquefaction) is indicated by full line arrows having a single break and the daytime flow direction (natural gas vaporization) is indicated by having multiple dashed lines.

At night, the excess CO -free, dry natural gas is introduced via conduit 40 at 8 bar, compressed in compressor 41 to 50 bar, cooled to about 153K, liquefied in flow path 42 of heat exchanger 43, and expanded via valve 44 to the pressure of the storage tank 45, i.e. approximately 1 bar. Unliquefied gas, as well as the portion vaporized by the effect of ambient heat, are with drawn via conduit 46, warmed in flow path 47, brought to the pressure of the arriving natural gas by the compressor 48, and then further compressed and liquefied together with the last-mentioned natural gas.

The refrigeration for liquefaction is derived from the liquid propane stored in the storage tank 49 at a temperature of about 132K. The propane is conveyed, with the aid of pump 50, through the flow path 51 of the heat exchanger 43 and during this step first warmed to 215K. The major portion of the liquid propane, i.e. about 73 percent, is then branched off via line 52 and fed, by way of valve 53, into the storage tank 54; the remainder is warmed to ambient temperature and fed, via conduit 55, to the storage tank 56 which is under a pressure of about 11 bar.

During the period liquid propane is pumped from tanks 54 and 56, the hydrogen in the vapor spaces of tanks 49 and 54 flows in the reverse direction. Thus, hydrogen displaced from the storage tank 54 by the liquid propane flowing into this tank, passes via conduit 57 into the inert gas flow path 58 of the heat exchanger 43. From the heat exchanger 43, it is passed via conduit 60, together with hydrogen from vessel 59 into the vapor space of the vessel 49 from which propane is being discharged. This hydrogen thereby equalizes the difference in volume between the amount of liquid withdrawn from the vessel 49 and the amount of liquid fed into the vessel 54. In this connection, it is to be taken into account that the equilibrium concentration of propane in the gaseous phase is only about 0.03 mole percent at 132K, but approximately 35 mole percent at 215K. The gaseous mixture leaving the vessel 54 thus consists of about 35 mole percent of propane; however, 'most of this propane condenses out again in vessel 49.

During the day, the natural gas is again vaporized for consumption. Specifically, it is pumped to the pipeline pressure, i.e. about 8 bar in pump 61 vaporized and warmed in flow path 42, and then withdrawn via conduit 49. To prevent the occurrence of subatmospheric pressure in the tank 45 during this step, a sufficient amount of natural gas is recycled thereto by branching off a portion (e.g. about 0.4 M01 percent) via valve 62, and passing same through flow path 47 and conduit 46. At the same time, liquid propane is withdrawn from the storage tank 56 via conduit 55, cooled in flow path 51, and mixed with the propane pumped out of vessel 54 at 215K via pump 63. The mixture is then further cooled in exchanger 43 to l32K and then fed via valve 64 into the tank 49. During this procedure, the hydrogen is withdrawn from the vapor space of vessel 49 via conduit 60, is partially warmed in the inert gas flow path 58, and is then passed, in part, via conduit 57 into the vapor space of the storage vessel 54 and, in part,

after further warming, into the vessel 59. The hydrogen in vessel 54 becomes saturated with propane and remains in that state until the beginning of the subsequent liquefying period. The resultant slight temperature drop can be neglected.

Assuming a plant design capable of a liquefaction rate of 100,000 Nm /h. of natural gas, there will be in a 10 hour period a total of 1,700 m of liquid natural gas fed to the storage tank 45, and 22,800 Nm lh. of natural gas warmed and recompressed as flash gas via conduit 46. The required cold is provided by 2,900 tons (4,140 m) of liquid propane which is withdrawn from the storage tank 49 and warmed to 215K. Twenty two hundred tons (3,630 m) of propane is then fed into the storage tank 54; 700 tons (1,460 m) is warmed to ambient temperature and stored in tank 56. The heat balance of the entire process is hardly affected by the above-described circulation of the inert gas. The required amount of hydrogen is 0.6 ton (6,700 Nm).

With reference to FIG. 5, a solid cryophore is utilized, rather than a liquid cryophore in a reduced amount for cooling the natural gas or methane to the median temperature and for warming to ambient temperature. The mode of operation during the nighttime is as in FIG. 4, illustrated by arrows drawn in full lines, and the operation during the daytime by dashed arrows.

According to FIG. 5, the methane supplied during the night from the pipeline 70 at 10 bar is compressed in compressor 71 to 25 bar and cooled to about 190K in heat exchange with packing in the regenerator 72 and with the gaseous methane flowing within the pipe coils 73, said gaseous methane being obtained from the storage tank 74. In the countercurrent heat exchanger 75, the liquefaction of the methane is effected by heat exchange with the liquid ethane which serves as the liquid cryophore. The ethane is withdrawn via pump 77 from the tank 76, under a pressure of about 0.1 bar and at 150K, is then warmed to 185K, and stored at this temperature in the tank 78. During this step, a pressure of 1.1 bar is realized in the tank 78.

The liquid methane exiting from the heat exchanger 75 is subcooled in heat exchanger 79 by the methane escaping in the gaseous phase from tank 74 via conduit 80, and is finally expanded, via valve 81, into the tank 74 which is under approximately atmospheric pressure. The thus-produced vapor and the proportion of liquid methane vaporized by the effect of heat are conducted as flash gas via conduit 80 through the heat exchanger 79 and the pipe coils 73 and, during this step, warmed to ambient temperature. This warmed methane vapor is brought to pipeline pressure in compressor 82, then reintroduced into the compressor 71 together with the methane coming from the pipeline 70.

During the revaporization in the daytime, liquid methane is withdrawn from tank 74 by means of pump 83, brought to 10 bar, and vaporized in heat exchanger 74 countercurrently to the ethane entering from tank 78 at a temperature of 185K. During this step, the ethane is cooled to 150K and is fed, via valve 84, to the tank 76 and stored therein. During the withdrawal of liquid methane, the pressure drops within the tank 74, and is compensated for only partially by the flash gas produced by the effect of heat. For this reason, and for reasons of heat balance in the regenerator 72, a portion of the vaporized methane, e.g. about 10 to percent, is branched ofi via conduit 85 and fed to the tank 74 during the withdrawal of liquid methane. The major portion of the vaporized methane is warmed to ambient temperature in the regenerator 72 and discharged via line 70.

In place of ethane, it is also possible to use as the cryophore a liquid consisting of 93 mole percent of ethylene and 7 mole percent of methane. The tank, operated at K, is then under a pressure of 1 atm. abs.; the gaseous phase in the vapor space contains 28 mole percent of ethylene. In the tank operated at K, a pressure of 6 atrn. abs. and a vapor composition of 50 mole percent of ethylene and 50 mole percent of methane are obtained.

A paper entitled Cryophore for the Short-Term Storage of LNG provides an economic work-up and rough cost estimate of the present invention and is authored by Wolfgang Forg in the published proceedings of the Second International Conference on Liquefied Natural gas held on Oct. 19-23, 1970. The sponsors of this conference were the International Gas Union, International Institute of Refrigeration and Institute of Gas Technology Chicago. The paper should be available from any one of the sponsors or the author (employed by Linde A6. of Hoellriegelskreuth, West Germany) or by Maison de la Chemie, Paris, France. Said paper is incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

In the following claims, the term natural gas is used in a generic sense to cover methane or natural gas, in accordance with this invention.

What is claimed is:

1. In a process of storing natural. gas, comprising liquefaction of the natural gas during low demand periods and revaporization of natural gas during high demand periods, wherein a liquid cryophore is employed to store heat liberated during the cooling and liquefaction of the natural gas, and to release heat during the vaporization and warming up of the natural gas, said cryophore being present in the liquid state in every phase of the process,

the improvement comprising employing said liquid cryophore in a given amount below a median temperature level during the cooling and liquefaction, and during the vaporization and warming of the natural gas and employing said liquid cryophore in an amount substantially less than said given amount at above said median temperature, said median temperature being approximately that temperature at which said natural gas commences to liquefy.

2. A process as defined by claim 1 wherein said median temperature is plus or minus 30K from the actual dew point of the natural gas.

3. A process as defined by claim 1 wherein said median temperature is the actual dew point of the natural gas.

4. A process as defined by claim 1 wherein said median temperature is plus or minus lK from the actual dew point of the natural gas.

5. A process as defined by claim 4, liquid cryophore being a mixture of 80-95 mol percent ethane, and 5-20 percent of a gas selected from the group consisting of methane and argon.

6. A process according to claim 1, comprising storing liquid cryophore at a low temperature above the storage temperature of the liquid natural gas, and during the cooling and liquefaction of the natural gas heating said liquid cryophore to the median temperature in heat exchange with incoming natural gas to be liquefied storing a portion of resultant liquid cryophore at said median temperature; warming the remainder of the liquid cryophore to approximately ambient temperature by further heat exchange with the natural gas; storing said remainder of said liquid cryophore at ambient temperature; and during the vaporization and warming of the natural gas, cooling the liquid cryophore stored at the ambient temperature to the median temperature by heat exchange with the natural gas, combining resultant cooled liquid cryophore with the liquid cryophore stored at the median temperature, and further cooling resultant combined liquid cryophore streams to said low temperature by further heat exchange with the natural gas, and storing resultant liquid cryophore at said low temperature.

7. A process as defined by claim 6, said liquid cryophore being a mixture of ethylene and methane.

8. A process as defined by claim 6, said liquid cryophore being a solution of liquids having different boiling points, the liquid having the lower boiling point having a vapor pressure of at least one atmosphere absolute at said low temperature above the temperature of natural gas.

9. A process according to claim 6, said liquid cryophore stored at the low temperature being maintained separate from the liquid cryophore stored at the median temperature by means of a insulating cover floating on the surface of said low temperature cryophore.

10. A process according to claim 6, further comprising maintaining at least one of (a) the liquid cryophore stored at the median temperature and (b) the liquid cryophore at said low temperature, under an atmosphere of inert gas.

11. A process according to claim 10, said liquid cryophore being maintained separate from the inert gas atmosphere by means of a cover floating on the surface of the liquid.

12. A process according to claim l0, further comprising cooling said inert gas, during the cooling and liquefaction of natural gas, from the ambient temperature to approximately the median temperature in heat exchange with the liquid cryophore to be warmed to ambient temperature.

13. A plant suitable for storing natural gas, said plant comprising a heat exchanger (43) having a flow path (42) for natural gas and a flow path (51) for liquid cryophore, a storage tank (45) for liquid natural gas, said storage tank being in communication with the cold end of the natural gas flow path (42), three storage vessels for liquid cryophore,, the first (49) being in communication with the cold end of the liquid cryophore flow path (51), the second (54) being in communication with a central section of the liquid cryophore flow path (51), and the third (56) being in communication with the warm end of the liquid cryophore flow path (51), and conduit means for effecting said communications.

14. A plant according to claim 13, further comprising insert gas tank (59) in communication via the warm end of an inert gas flow path (58) in said heat exchanger (43) with the heads of the first and of the second storage tanks (49 and 54), the head of the first storage tank (49) being in communication with the cold end of the inert gas flow path (58) and the head of the second storage tank (54) being in communication with a central section of the inert gas flow path (58).

15. In a process of storing natural gas, comprising liquefaction of the natural gas during low demand periods and revaporization of natural gas during high demand periods, wherein a cryophore is employed to store heat liberated during the cooling and liquefaction of the natural gas, and to release heat during the vaporization and warming up of the natural gas,

the improvement comprising employing a liquid cryohphore below a median temperature level during the cooling and liquefaction, and during the vaporization and warming of the natural gas, said cryophore being present in the liquid state in every phase of the process at the temperature employed; and at above said median temperature employing a solid cryophore, said median temperature being approximately that temperature at which said natural gas commences to liquefy.

16. Apparatus for storing natural gas comprising a pipeline for natural gas in communication with a regenerator (72), a heat exchanger having a first flow path for natural gas, said first flow path being down stream of and in communication with said regenerator and said pipeline, and a second flow path for liquid cryophore, a storage tank (74) for liquid natural gas upstream of and in communication with said first flow path, said second flow path of said heat exchanger (75) being in communication at the cold end thereof with a vessel (76) for the liquid cryophore stored at a low temperature, and at the warm end thereof to a vessel (78) for the liquid cryophore stored at a median temperature. 

2. A process as defined by claim 1 wherein said median temperature is plus or minus 30*K from the actual dew point of the natural gas.
 3. A process as defined by claim 1 wherein said median temperature is the actual dew point of the natural gas.
 4. A process as defined by claim 1 wherein said median temperature is plus or minus 10*K from the actual dew point of the natural gas.
 5. A process as defined by claim 4, liquid cryophore being a mixture of 80-95 mol percent ethane, and 5-20 percent of a gas selected from the group consisting of methane and argon.
 6. A process according to claim 1, comprising storing liquid cryophore at a low temperature above the storage temperature of the liquid natural gas, and during the cooling and liquefaction of the natural gas heating said liquid cryophore to the median temperature in heat exchange with incoming natural gas to be liquefied storing a portion of resultant liquid cryophore at said median temperature; warming the remainder of the liquid cryophore to approximately ambient temperature by further heat exchange with the natural gas; storing said remainder of said liquid cryophore at ambient temperature; and during the vaporization and warming of the natural gas, cooling the liquid cryophore stored at the ambient temperature to the median temperature by heat exchange with the natural gas, combining resultant cooled liquid cryophore with the liquid cryophore stored at the median temperature, and further cooling resultant combined liquid cryophore streams to said low temperature by further heat exchange with the natural gas, and storing resultant liquid cryophore at said low temperature.
 7. A process as defined by claim 6, said liquid cryophore being a mixture of ethylene and methane.
 8. A process as defined by claim 6, said liquid cryophore being a solution of liquids having different boiling points, the liquid having the lower boiling point having a vapor pressure of at least one atmosphere absolute at said low temperature above the temperature of natural gas.
 9. A process according to claim 6, said liquid cryophore stored at the low temperature being maintained separate from the liquid cryophore stored at the median temperature by means of a insulating cover floating on the surface of said low temperature cryophore.
 10. A process according to claim 6, further comprising maintaining at least one of (a) the liquid cryophore stored at the median temperature and (b) the liquid cryophore at said low temperature, under an atmosphere of inert gas.
 11. A process according to claim 10, said liquid cryophore being maintained separate from the inert gas atmosphere by means of a cover floating on the surface of the liquid.
 12. A process according to claim 10, further comprising cooling said inert gas, during the cooling and liquefaction of natural gas, from the ambient temperature to approximately the median temperature in heat exchange with the liquid cryophore to be warmed to ambient temperature.
 13. A plant suitable for storing natural gas, said plant comprising a heat exchanger (43) having a flow path (42) for natural gas and a flow path (51) for liquid cryophore, a storage tank (45) for liquid natural gas, said storage tank being in communication with the cold end of the natural gas flow path (42), three storage vessels for liquid cryophore,, the first (49) being in communication with the cold end of the liquid cryophore flow path (51), the second (54) being in communication with a central section of the liquid cryophore flow path (51), and the third (56) being in communication with the warm end of the liquid cryophore flow path (51), and conduit means for effecting said communications.
 14. A plant according to claim 13, further comprising insert gas tank (59) in communication via the warm end of an inert gas flow path (58) in said heat exchanger (43) with the heads of the first and of the second storage tanks (49 and 54), the head of the first storage tank (49) being in communication with the cold end of the inert gas flow path (58) and the head of the second storage tank (54) being in communication with a central section of the inert gas flow path (58).
 15. In a process of storing natural gas, comprising liquefaction of the natural gas during low demand periods and revaporization of natural gas during high demand periods, wherein a cryophore is employed to store heat liberated during the cooling and liquefaction of the natural gas, and to release heat during the vaporization and warming up of the natural gas, the improvement comprising employing a liquid cryohphore below a median temperature level during the cooling and liquefaction, and during the vaporization and warming of the natural gas, said cryophore being present in the liquid state in every phase of the process at the temperature employed; and at above said median temperature employing a solid cryophore, said median temperature being approximately that temperature at which said natural gas commences to liquefy.
 16. Apparatus for storing natural gas comprising a pipeline (70) for natural gas in communication with a regenerator (72), a heat exchanger (75) having a first flow path for natural gas, said first flow path being down stream of and in communication with said regenerator and said pipeline, and a second flow path for liquid cryophore, a storage tank (74) for liquid natural gas upstream of and in communication with said first flow path, said second flow path of said heat exchanger (75) being in communication at the cold end thereof with a vessel (76) for the liquid cryophore stored at a low temperature, and at the warm end thereof to a vessel (78) for the liquid cryophore stored at a median temperature. 